The present invention relates to a multi-layer heat-shrinkable tubular sleeve according to claim 1, to a cable termination comprising a multi-layer heat-shrinkable tubular sleeve according to claim 13, to a cable joint comprising a multi-layer heat-shrinkable tubular sleeve according to claim 14 and to a method for producing a multi-layer heat-shrinkable tubular sleeve according to claim 15.
It is known in the state of the art to provide energy cables with a voltage rating of 6 kV or higher with cable insulations comprising inner and outer conductive layers for electrical stress minimization. In order to create connections in cable terminations or cable joints, the outer conductive layer has to be removed. This creates an area of enhanced electric fields at the edge of the outer conductive layer. In order to guarantee safe operation, that field has to be attenuated using appropriate means, such that no discharges can occur. This attenuation of the electric field is referred to as electrical stress control.
It is known in the state of the art to use heat-recoverable tubular sleeves with suitable material parameters, especially suitable electrical conductivity and permittivity, to grade the electric field. In the state of the art such tubular sleeves are used in cable terminations and cable joints. These sleeves are typically shrunk into position around the ends of cables that have been electrically connected together.
During installation of these products an electrical interface between the cable and the product is created. It is known in the state of the art that the quality of the electrical interface improves when a radial pressure exerted by the tubular sleeve is increased. Advancing optimization of the cables, decreasing wall thicknesses and tightened inspection requests require a further improvement of such interfaces.
It is also known in the state of the art to use several separate heat-recoverable tubular sleeves on top of each other to provide several functions as stress control, insulation or durability against environmental stresses. This however requires additional work steps during the installation of the cable connections.
The EP 1 702 391 B1 describes a heat-shrinkable jointing with an inner layer, a mid-layer and an outer layer. The outer layer may comprise a small quantity of a high density polyethylene.
From the U.S. Pat. No. 5,013,894 it is known to provide a tubular sleeve comprising a conductive polymeric material with two or more elongate electrodes extending at least part way along the length of the sleeve. The electrodes can be connected to a source of electrical power to cause electrical current to flow between the electrodes around the circumference of the sleeve in order to heat up and shrink the sleeve.